1. Field of the Invention
This invention relates to bottoms for filter beds and more particularly to bottoms for filters in which a backwash fluid is periodically introduced from beneath the filter bed.
2. Prior Art
It is common practice to periodically backwash a filter bed by introducing a fluid in the form of a gas, a liquid or a combination of the two through the bottom of the filter. It is known to introduce the backwash fluid through nozzles served by pipes laid in channels in the filter bottom. It is also known to provide a false bottom of porous material or with strainers therein through which the backwash fluid may be introduced into the filter bed. A common problem with these prior art constructions is that the nozzles, porous material or strainers can become clogged over a period of time with material which cannot be removed by the backwashing. This condition necessitates that the filter be torn down for cleaning or replacement of the clogged parts.
U.S. Pat. No. 2,710,692 suggests that backwash gas emitted from apertures in pipes laid below the filter bed be collected in inverted channels laid transverse to and above the pipes. The gas is said to disperse along the channel members and escape uniformly through slots in the lower portion of the channels. The gas then percolates upward and passes through longitudinal spaces between elongated members forming a false bottom which supports the filter media.
In another type of filter bottom, a number of concrete blocks, called M blocks because of the cross section of a channel extending lengthwise through the blocks, are placed end-to-end in parallel rows on the floor of the filter bed with the open side facing downward to provide an array of parallel, elongated chambers below a false bottom. Backwash gas introduced into selected chambers by pipes laid therein percolates upward and is diverted laterally by the M-shaped roof of the chamber to bores in the side walls of the blocks. Projections on the sides of the blocks maintain spacing between adjacent rows of blocks such that the backwash gas may escape from the side bores. Transverse openings at the bottom of the side walls of each block interconnect the parallel, elongated chambers. This arrangement provides a strong floor for the filter media which does not easily clog, however, the M blocks are expensive and difficult to fabricate.